From this one-particle picture, we expect that a system with two or more species with different Stokes' velocity may separate during the process of sedimentation. However, reality is not necessarily that simple. In a suspension of a finite concentration, the kinetics of sedimentation are strongly influenced by hydrodynamic interactions between particles [1]}, [2]}, and direct attractive interactions between the sedimenting particles can have a drastic effect and even qualitatively alter the simple picture above [3]}. For example, in a colloid-polymer mixture (C+P), complex couplings between sedimentation and the polymer concentration come into play [4]}. The polymer chemical potential drives the depletion attraction between the colloids, yet the polymer free volume is itself coupled to a state point of the system [5]}. Reflecting this feature, predictions from free volume theory suggest that highly non-intuitive behaviour occurs even in equilibrium in the form of a colloidal liquid floating above a colloidal gas [6]}. Furthermore, it was shown that a system of C+P undergoes flocculation and then each flock settles much faster than individual particles, and can couple to phase ordering such as crystallisation during sedimentation [7]}. In the sticky-sphere limit (where the size of the polymer is very small compared to the size of the colloid) flocculation can lead to dense colloidal crystals or kinetically arrested gels[8]}.
